This invention belongs to the technical field of resin-coated steel pipe with superior mechanical strength, including the ability to slide, which uses, as its outer coating resin, a crystalline engineering plastic notably superior in mechanical strength, including the ability to slide, and in heat resistance. More particularly, the invention relates to a resin-coated steel pipe with superior mechanical strength, including the ability to slide, the crystalline engineering plastic of which greatly improves the adhesiveness of the steel pipe, making the pipe suited for use as a spline shaft used in live-roller conveyors for light or medium loads, and the like.
Crystalline engineering plastics such as PBT (polybutylene terephthalate) resin and nylon resin are notably superior in mechanical strength, including the ability to slide, and in heat resistance. Thus, if such plastics can be used as the outer coating resin of resin-coated steel pipes, then it is possible to obtain resin-coated steel pipes superior in their ability to slide, as well as in their durability. Accordingly, there are remarkable advantages to such steel pipes, and it is expected that there will also be great demand for the same. Resin-coated steel pipes are most suitable for use as members requiring a significant ability to slide, such as spline shafts used in live-roller conveyors for light or medium loads.
When using a crystalline engineering plastic as the outer coating resin of a resin-coated steel pipe, there has hitherto been no suitable adhesive that was able to satisfactorily bond the crystalline engineering plastic to a thin-walled steel pipe. The only available measure was to coat the thin-walled steel pipe with a thin film of a modified polyolefin-based adherent polymer and then bond the crystalline engineering plastic to the surface of the polymer.
However, the resin-coated steel pipe constructed by bonding a crystalline engineering plastic to the thin-walled steel pipe with an adherent polymer is problematic in that water is liable to seep into the interface between the thin-walled steel pipe and the adherent polymer, causing the separation of the coated resin (crystalline engineering plastic). Especially in cases where the resin-coated steel pipe is used outdoors, or in the field of agriculture or fisheries, regular contact with water will result in accelerated separation of the coated resin, thus impairing the practicality of the resin-coated steel pipe for such applications.
Also, although crystalline engineering plastics are superior in mechanical strength, including the ability to slide, as well as in heat resistance, they have a low melt viscosity and a large shrinkage factor. Consequently, a disadvantage also arises in that shaping stability, and thus dimensional accuracy, is low, lowering production efficiency.
Moreover, crystalline engineering plastics are in themselves expensive and are not economical materials. In the present circumstances, therefore, crystalline engineering plastics should be coated to the smallest possible thickness, thereby reducing the amount of the expensive resins used.
As stated above, it is extremely difficult to use a crystalline engineering plastic as the outer coating resin of a resin-coated steel pipe, and no such resin-coated steel pipe has been put to practical use yet.
Currently, therefore, the outer resin coating of resin-coated steel pipes is formed using synthetic resins, such as acrylate acrylic styrene (AAS), acrylonitrile-butadiene-styrene (ABS), Acrylonitrile-ethylene-styrene (AES) and Poly(ethylene terephthalate) glycol (PETG), which are inferior to crystalline engineering plastics in mechanical strength, including the ability to slide, and in heat resistance, but permit the use of a rubber-based adhesive which ensures excellent water resistance of the adhesion interface between the thin-walled steel pipe and the coated resin, and does not entail the separation of the coated resin.
A first object of the present invention is to provide a resin-coated steel pipe with superior mechanical strength, including the ability to slide, in which crystalline engineering plastic has greatly improved adhesiveness with respect to a thin-walled steel pipe, and thus is free from problems of separation.
A second object of the present invention is to provide a resin-coated steel pipe with superior mechanical strength including slidability, which can be more economically manufactured with improved production efficiency.